Well, personally I'm into wireless technology so I would of course recommend some epic bluetooth stereo system. (I have no specific system in mind, but I'm sure there are some great ones out there) It doesn't really need to be designed to hold the Droid, if it's a bluetooth stereo that is set up for the Droid to stream music to. Admittedly, though, this might be kind of expensive compared to wired systems, and you would need to find a bluetooth system that doesn't sound like sh*t.
Alternatively I'd recommend maybe downloading gmote from the Marketplace. This is an app that, if you download the respective server app for your computer, can communicate with your PC and start playing music files/videos on your PC. I have my PC (which I usually keep on all the time) hooked up to a sound system, so when I want to listen to music I just open the gmote app on my Droid, and tell it to start playing. The Droid uses my wifi connection to tell the PC to play music. This comes in handy when you have a large music collection on your PC, that won't necessarily all fit on your Droid's memory stick.